The present invention relates to a cold cathode electron source which is sought after for use in applications such as electron-ray excited lasers, elements for a flat type display, and ultra-fast micro vacuum elements. More particularly, the present invention also relates to a semiconductor-applied field emission type electron source which can be integrated and requires a low voltage and a method for producing the same.
Semiconductor micro-processing technology has made progress so that a micro cold cathode structure can be constructed. This leads to vigorous development of vacuum micro electronics technology. Such a micro cold cathode structure obtained by this technology is thought to achieve a flat-type electron emission characteristic and a high level of current density. In particular, this micro cathode structure is therefore believed to serve as an electron source of a next-generation flat display. Further, the structure has an operating temperature in a range wider than that of a liquid crystal display mode such as a TFT-LCD. For this reason, such a cathode structure is thought to be useful in a display which is carried on vehicles and is required to be resistant to harsh environments.
A reduction in operating voltage, a stable electron emission characteristic, a long life characteristic, and the like are required for these electron sources when used for a flat display. In particular, the stable electron emission characteristic is directly involved in brightness which is basic to the performance of a display, regarded as an important technological target.
To obtain this goal, a method in which a resistor layer is inserted inside the electron source, a method in which a constant-current circuit is incorporated into the electron source and the like have been proposed.
Hereinafter, the configuration of a field emission cold cathode device described in Japanese Laid-Open Publication No. 8-87957 will be described with reference to FIGS. 8(a) and 8(b). This first conventional example adopts a principle such that a constant emitted electron flow amount of a field emission cathode element is obtained using the constant current characteristic of a field effect transistor (FET). FIG. 8(a) is a cross-sectional view of a part of a silicon substrate on which a field emission cathode element and a FET are provided. FIG. 8(b) is a circuit configuration diagram showing an electrical equivalent circuit of the part including the field emission cathode element.
In FIGS. 8(a) and 8(b), reference numeral 810 denotes a field effect transistor (FET); 801 a p-type silicon substrate; 802 a first n-type layer which serves as the source of the FET 810; 803 a cone-shaped emitter of the field emission cathode element; 804xe2x80x2 a part of an isolating layer (SiO2 layer) 804, the part functioning as a gate isolating layer of the field emission cathode element; 805 a gate layer of the field emission cathode element; 806 a second n-type layer which serves as the drain of the FET 810; 807 the source electrode of the FET 810; 808 the gate electrode of the FET 810; 809 the anode of the field emission cathode element; 811 a source resistor; 812 a gate voltage source (voltage value Vg); 813 an anode voltage source (voltage value Va); and 814 a gate-to-source control voltage source (voltage value Vgs).
As shown in FIG. 8(b), the field emission cathode element has a structure of a triode including the anode (A) 809, the gate (G) 805, and the emitter (E) 803. The drain-source path and source resistor 811 of the FET 810 are connected in series between the emitter (E) 803 and the ground.
In this triode, the anode (A) 809 is connected to the anode voltage source 813 which generates the anode voltage Va. The gate (G) 805 is connected to the gate voltage source 812 which generates a fixed gate voltage Vg. In the FET 810, the gate 808 is connected to the gate-to-source control voltage source 814 which generates a variable gate-to-source control voltage Vgs.
In the field emission cathode element for use in a field emission cathode device, a predetermined anode voltage Va and a predetermined gate voltage Vg are applied to the anode 809 and the gate 805, respectively. When a predetermined value of gate-to-source voltage Vgs is then applied to the gate 808 of the FET 810, emitted electron flow is generated from the emitter 803 without heating the emitter 803. In this case, the amount of the emitted electron flow by the field emission cathode element is not controlled by the fixed gate voltage Vg applied to the gate 805, but is controlled by the variable gate-to-source control voltage Vgs applied to the gate 808 of the FET 810 connected to the emitter 803. In other words, an appropriate gate-to-source control voltage Vgs applied to the gate 808 of the FET 810 allows the FET 810 to operate in a constant current region.
As described above, the amount of the emitted electron flow from the emitter caused by field emission is determined by a characteristic of the FET which is connected in series to the emitter and has a function of supplying electrons which will be emitted. Therefore, optimization of the FET design allows predetermination of the operating conditions and field emission electron flow amount of the FET. In particular, when the field emission is performed in the saturated operating region of the FET, the field emission is free from factors of instability of the emitter itself. As a result, an extremely stable and accurately controlled field emission current amount can be obtained.
Among specifications required for a cold cathode is a high definition which is a very important factor for a display application. In general, in the case of a micro-chip-type cold cathode structure, the emitter emits electrons at a predetermined divergent angle. This is likely to be detrimental. A structure using a focus electrode has been proposed to provide a means for preventing the divergence of an electron path. FIG. 9 shows a configuration example of an FED using such a structure, as a second conventional example, which is disclosed in Japanese Laid-Open Publication No. 10-74473.
In this FED, a second gate electrode (focus electrode) is formed for each emitter. This gate electrode receives a potential which is negative relative to a first gate electrode (extraction gate electrode) so as to converge electrons emitted from the emitter.
In other words, in FIG. 9, reference numeral 91 denotes an insulating layer. An insulating layer 93 is further provided on a gate electrode (extraction electrode) 92. On the insulating layer 93 a second gate electrode (focus electrode) 94 which has a round opening portion is provided. In this conventional example, the second gate electrode (focus electrode) 94 is provided in such a manner as to surround each emitter 95. This second gate electrode (focus electrode) 94 is set to a potential lower than the first gate electrode (extraction gate electrode) 92 so that electrons emitted from the emitter are affected by a lens action having a convergence effect. This causes the electron beam paths to converge.
However, the field emission type cathode element of the above-described first conventional example can control the field emission current to be stable for a short time period. In some operating condition, the stability is not secured for a long time period.
Further, whereas the field emission type display device of the second conventional example has a function of converging electron beams, the amount of electrons emitted from the emitter is adversely reduced.
The present invention is provided to solve the above-described problems. Objectives of this invention are as follows: (1) to obtain a field emission type electron source structure which achieves highly reliable operation required for next-generation displays; (2) to obtain a field emission type electron source structure which achieves high density and stable operation for high definition; and (3) to obtain a field emission type electron source structure having a beam convergence action which allows higher definition for a display application.
According to one aspect of the present invention, a field emission type electron source device includes a field emission electron source portion including an extraction electrode provided on a p-type silicon substrate via an insulating film and having an opening portion at a position corresponding to a region where a cathode is provided; and a cathode portion provided on the p-type silicon substrate and at a position corresponding to the opening portion of the extraction electrode; and an n-channel field effect transistor portion provided on the p-type silicon substrate, corresponding to the field emission electron source portion. The field emission electron source portion is provided in a drain region of the field effect transistor portion; and a control voltage is applied to a gate electrode of the field effect transistor portion to control a field emission current from the field emission electron source portion. The drain region includes at least two wells having different impurity concentrations. Of the at least two wells, one well having a low impurity concentration is provided at an end of the drain region which contacts a channel region of the field effect transistor portion.
For example, as the impurity elements the drain region may include at least two n-type impurity elements having different thermal diffusion speeds in the silicon substrate.
In one embodiment, as the impurity elements, the drain region includes phosphorous, having a fast thermal diffusion speed and arsenic, having a slow thermal diffusion speed in the silicon substrate.
According to another aspect of the present invention, a field emission type electron source device includes a field emission electron source portion including an extraction electrode provided on a p-type silicon substrate via an insulating film and having an opening portion at a position corresponding to a region where a cathode is provided; and a cathode portion provided on the p-type silicon substrate and at a position corresponding to the opening portion of the extraction electrode; and an n-channel field effect transistor portion provided on the p-type silicon substrate, corresponding to the field emission electron source portion. The field emission electron source portion is provided in a drain region of the field effect transistor portion. A control voltage is applied to a gate electrode of the field effect transistor portion to control a field emission current from the field emission electron source portion. The gate electrode of the field effect transistor portion has a shape including portions having at least two different gate widths; and a part of the gate electrode is provided in such a manner as to cover an end of the drain region.
According to further another aspect of the present invention, a field emission type electron source device includes a field emission electron source portion including an extraction electrode provided on a p-type silicon substrate via a first insulating film and having an opening portion at a position corresponding to a region where a cathode is provided; and a cathode portion provided on the p-type silicon substrate and at a position corresponding to the opening portion of the extraction electrode; and an n-channel field effect transistor portion provided on the p-type silicon substrate, corresponding to the field emission electron source portion. The field emission electron source portion is provided in a drain region of the field effect transistor portion; and a control voltage is applied to a gate electrode of the field effect transistor portion to control a field emission current from the field emission electron source portion. A gate insulating film is provided between the gate electrode of the field effect transistor and the p-type silicon substrate. The gate insulating film includes a film thinner than the first insulating film, the first insulating film being provided between the extraction electrode and the p-type silicon substrate. The gate insulating film is buried with the first insulating film.
The gate insulating film may include a thermally oxidized silicon film, provided by a step of thermal oxidization for sharpening treatment for sharpening a tip of the cathode portion of the field emission electron source portion.
According to still another aspect of the present invention, a field emission type electron source device includes a field emission electron source portion including an extraction electrode provided on a p-type silicon substrate via an insulating film and having an opening portion at a position corresponding to a region where a cathode is provided; and a cathode portion provided on the p-type silicon substrate and at a position corresponding to the opening portion of the extraction electrode; and an n-channel field effect transistor portion provided on the p-type silicon substrate, corresponding to the field emission electron source portion. The field emission electron source portion is provided in a drain region of the field effect transistor portion. A control voltage is applied to a gate electrode of the field effect transistor portion to control a field emission current from the field emission electron source portion. The field emission type electron source device further includes a shield electrode made of the same material of that of the gate electrode of the field effect transistor portion, and the shield electrode is provided in such a manner as to cover a channel region of the field effect transistor which is not covered with the gate electrode.
Preferably, the shield electrode is held at the same potential as that of the p-type silicon substrate, and the shield electrode has a function of blocking an external field, which is not caused by the gate electrode, from affecting the channel region.
According to still another aspect of the present invention, a field emission type electron source device includes a field emission electron source portion including an extraction electrode provided on a p-type silicon substrate via an insulating film and having an opening portion at a position corresponding to a region where a cathode is provided; and a cathode portion provided on the p-type silicon substrate and at a position corresponding to the opening portion of the extraction electrode; and an n-channel field effect transistor portion provided on the p-type silicon substrate, corresponding to the field emission electron source portion. The field emission electron source portion is provided in a drain region of the field effect transistor portion. A control voltage is applied to a gate electrode of the field effect transistor portion to control a field emission current from the field emission electron source portion. The drain region of the field effect transistor portion is provided in a source region of the field effect transistor portion in such a way to be surrounded by the source region. The gate electrode of the field effect transistor portion is positioned symmetrical in a plane with respect to the cathode of the field emission electron source portion.
For example, the drain region includes a p-type conductive layer.
An outer portion of the drain region may contact the channel region of the field effect transistor portion. The outer region of the drain region and an inner portion of the source region may have a shape of concentric circles.
At least a part of the gate electrode provided between the source region and the drain region may have a shape of a symmetrical circular arc.
For example, first voltage Vex applied to the extraction electrode of the field emission electron source portion and second voltage Vg applied to the gate electrode of the field effect transistor portion have a relationship such that Vg less than Vex.
According to this invention, the drain end of the FET, into which high field intensity is concentrated, includes a well having a low impurity concentration. As a result, extreme concentration of fields can be relaxed. Therefore, reliability of device operation can be significantly improved.
At least two n-type impurity elements having different thermal diffusion speeds in the silicon substrate are used as impurity elements in the drain region, thereby easily obtaining at least two n-type wells utilizing the difference in thermal diffusion speed.
When phosphorous having a fast thermal diffusion speed and arsenic having a slow thermal diffusion speed are used as the impurity elements, an n-negative well having a low impurity concentration and an n-positive well having a high impurity concentration can be easily provided.
According to this invention, in the field emission type electron source device, a part of the channel gate electrode covers the drain end region, thereby allowing a drain current flowing from, the source to the drain to be diffused in the drain end region. As a result, current density can be reduced.
According to this invention, in the field emission type electron source device, the thick insulating film for the extraction electrode requiring a high level of applied voltage and an insulating film for the field effect transistor in which a thin insulating film is required for low-voltage drive can be separated in terms of function. Further, the gate insulating film is buried by an insulating film, thereby making it possible to provide multilayer wiring. Accordingly, wiring for matrix drive can be easily provided.
When the gate insulating film is made of a thermally oxidized silicon film provided by thermal oxidization for sharpening the tip of the cathode of the field emission electron source portion, a thermal oxidization film having a high-quality which is accurately controlled is obtained. Therefore, a high level of reliability can be obtained, and the control of the FET can be carried out in high precision.
Further, according to this invention, in the field emission type electron source device, when the channel region of the field effect transistor portion is covered with the shield electrode, influences of an external field can be blocked. When the shield electrode is made of the same material as that of the gate electrode, a wiring step is simplified.
When an additional structure is provided such that the shield electrode is held at the same potential as that of the p-type silicon substrate so that influences of external fields other than that of the gate electrode can be blocked, the shield electrode is held at the same potential as that of the p-type silicon substrate so that the shield function against the external field can be secured.
According to this invention, in the field emission type electron source device, electrodes such as the gate electrode can be symmetrical in a plane with respect to the center of the drain, thereby easily obtaining convergence of electrons.
Furthermore, according to this invention, the step of doping an impurity into the drain region by ion injection can be simplified, thereby reducing manufacturing cost and preventing variation in a shape of the cathode due to the injection of ions to the cathode.
The outer portion of the drain and the inner portion of the source, both contacting the channel region of the field effect transistor portion, have a shape of a concentric circle. Therefore, uniform injection of carriers from the source region to the drain region can be obtained, thereby achieving a satisfactory characteristic of a transistor.
At least a part of the gate electrode, which is used to control the channel region, is provided between the source region and the drain region and has a shape of a symmetrical circular arc. Therefore, the shape of the electrode for convergence is symmetrical around the drain, thereby obtaining more uniform convergence.
The first voltage (Vex) applied to the extraction electrode of the field emission electron source portion and the second voltage (Vg) applied to the gate electrode of the field effect transistor portion have a relationship such that Vg less than Vex. Therefore, convergence of electrons can be secured.